Add 32-bit Core CI lane via test_groups.toml (#229)

Opt into SciML/.github arch=x86 testing with a matrix-only Core alias.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>

Author: ChrisRackauckas-Claude

test/test_groups.toml

@@ -4,5 +4,11 @@
 [Core]
 versions = ["lts", "1", "pre"]
 
+["Core 32-bit"]
+group = "Core"
+versions = ["1"]
+os = ["ubuntu-latest"]
+arch = "x86"
+
 [QA]
 versions = ["lts", "1"]